1. Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly to an implantable pacemaker that provides hysteresis in a dual chamber mode of operation that may be triggered by sensed natural rhythm occurring in either the atrium or the ventricle.
The heart is a pump that pumps life-sustaining blood through a patient's body. The heart achieves its pumping function through the contraction of its myocardial muscle tissue, which contraction squeezes the blood from one chamber of the heart to another chamber or to a specific location within the body. For example, as blood returns to the heart, after having circulated through the body, it is collected in the right atrium of the heart. Contraction of the right atrium pushes the blood held therein into the right ventricle. After a short delay, long enough to allow the blood to move from the right atrium to the right ventricle, the right ventricle contracts, forcing the blood to the patient's lungs. Blood returning from the lungs is collected in the left atrium. Contraction of the left atrium pushes the blood into the left ventricle. After a short delay, the left ventricle contracts, forcing the blood into the circulation system of the patient's body.
In a healthy heart, the right and left atria, as well as the right and left ventricles, contract simultaneously, with a short delay (e.g., 40-120 milliseconds) existing between the atrial contraction and the ventricular contraction, and a much longer delay (e.g., 350-1200 milliseconds) existing between the ventricular contraction and the next atrial contraction. It is this rhythm--of having the atria contract followed by having the ventricles contract, that is referred to as a heart "beat," or a cardiac "cycle." A typical heart may beat 85,000 to 100,000 times each day.
If the heart tissue is diseased or damaged, it may not be able to efficiently pump the blood through the body. Numerous types of maladies can occur, affecting either the ability of a given heart chamber to contract, or the timing of the myocardial muscle tissue contractions. Bradycardia, for example, is a condition of the heart where the heart beat slows to a rate that is considered insufficient to pump an adequate supply of blood through a patient's body. A heart rate of less than 50 beats per minute, for example, is usually considered as a bradycardia condition for most patients.
One common technique for treating bradycardia and other heart conditions is to implant a pacemaker in the patient. The pacemaker senses cardiac activity, i.e., heart beats, or contractions within a given heart chamber, and if the heart beats do not occur at a prescribed rate, then stimulation pulses are generated and delivered to an appropriate heart chamber, usually to either the right atrium or the right ventricle, in order to force the myocardial muscle tissue in those chambers of the heart to contract, thereby forcing the heart to beat at a faster rate or with a specified timed relationship.
In order to afford the heart every opportunity to beat on its own, i.e., to allow atrial and ventricular muscle tissue to contract naturally without external stimulation pulses, the circuits of the pacemaker define a period of time, generally referred to as the "escape interval," that is slightly longer than the period of time between heart beats of a heart beating at a minimal acceptable rate. For example, if the heart is beating at a rate of 50 beats per minute, the time period between consecutive heart beats is 1200 milliseconds. Thus, if it is desired that the heart rate never slow to a rate less than 50 beats per minute, the escape interval of the pacemaker is set to an appropriate value that causes a stimulation pulse to always be generated if more than 1200 milliseconds elapse since the last heart beat. If a heart beat occurs before 1200 milliseconds have elapsed, then the heart is beating at a rate faster than 50 beats per minute, and no stimulation pulse need be generated. Upon sensing such a "natural" (non-stimulated, sometimes referred to as "intrinsic") heart beat within the allotted time period, the escape interval is reset, and a new escape interval is started. A stimulation pulse will be generated at the conclusion of this new escape interval unless a natural heart beat is again sensed during the escape interval. In this way, stimulation pulses are generated "on demand," i.e., only when needed, in order to maintain the heart rate at a rate that never drops below the rate set by the escape interval.
The heart rate is monitored by examining the electrical signals that are manifest concurrent with the contraction of the cardiac muscle tissue in a given chamber of the heart. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the "QRS complex"). Because the ventricular muscle tissue is much more massive than the atrial muscle tissue, the R-wave is generally a much larger signal than the P-wave, and hence easier to detect. Advantageously, the sequence of electrical signals that represent P-waves, followed by R-waves (or QRS complexes) can be sensed by the pacemaker circuits from inside of or directly on the heart by using sensing leads implanted inside or on the heart, e.g., pacemaker leads. Such electrical signals representing internally-sensed P-waves and R-waves are referred to as the electrogram (EGM) of the heart. A dual chamber pacemaker advantageously includes means for sensing P-waves and/or R-waves, and hence means for monitoring the patient's EGM.
R-waves and/or P-waves are sensed by placing an electrode in contact with, or proximal to, the cardiac tissue of interest. Most pacemakers use the same electrode for sensing R-waves and/or P-waves as is used to deliver stimulation pulses to the corresponding ventricular and/or atrial cardiac tissue. Most modern pacemakers further include the ability, in combination with an external programming/display device in telecommunicative contact with the pacemaker, to display the EGM. A skilled cardiologist or other physician can determine a great deal about a patient's heart by simply studying the EGM of the patient. Further, the pacemaker circuits can be designed and programmed to automatically respond in an appropriate manner to various conditions manifest by the EGM.
In order to further enhance the ability of the heart to beat on its own without external stimulation, it is known in the art, when operating in certain single chamber pacing modes, to provide a longer escape interval in response to a sensed natural heart beat than is provided in response to an externally stimulated heart beat. One such single chamber pacing mode, used when sensing and pacing in the ventricle, is the VVI pacing mode. (See the next two paragraphs for a more complete description of the various pacing modes and how such modes are designated with a three or four letter code.) Thus, for example, if the pacing rate is set at 70 beats per minute (bpm), corresponding to an escape interval of 857 msec, and if no natural heart beats occur, a stimulation pulse is generated every 857 msec. Should a natural heart beat be detected, the escape interval is lengthened, e.g., by 10%, to 943 msec, in order to allow the heart to naturally beat at a rate that is slightly lower (.apprxeq.64 bpm) than the 70 bpm pacing rate. In this way, the natural rhythm of the patient is given a higher priority than is the forced (paced) rhythm set by the pacemaker. The changing of the escape interval in response to sensing a natural heart beat in this manner is referred to as "hysteresis."
As indicated, heretofore hysteresis has only been used in certain single chamber pacing modes. The pacing modes of a pacemaker, both single and dual chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an "O" indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting ("I") response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response wherein a stimulation pulse to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual ("D") response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the "inhibiting" occurring in one chamber of the heart and the "triggering" in the other.
To such three letter code, a fourth letter "R" may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled ("O" typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
Thus, for example, a DVI pacemaker is a pacer (note that throughout this application, the terms "pacemaker" and "pacer" are used synonymously) that paces in both chambers of the heart, but only senses in the ventricle, and that operates by inhibiting stimulation pulses when prior ventricular activity is sensed. Because it paces in two chambers, it is considered as a dual chamber pacemaker. A VVI pacer, on the other hand, is a pacer that paces only in the ventricle and senses only in the ventricle. Because only one chamber is involved, it is classified as a single chamber pacemaker. It should be noted that most dual chamber pacemakers can be programmed to operate in a single chamber mode. A DDDR pacer is a rate-responsive pacemaker that senses and paces in both chambers of the heart at a rate determined by a physiological sensor.
Heretofore, to applicant's knowledge, hysteresis has only been used in single chamber pacing modes, e.g., VVI, when cardiac activity is sensed in only one chamber of the heart. What is needed, therefore, is a hysteresis system that can be used in dual chamber pacing modes, such as DDD, or DDDR, or in other atrial tracking modes, such as VDD. The present invention advantageously provides such a system.